This invention relates to a process for producing chitin derivatives and/or chitosan derivatives with a crosslinked structure by adding purified water to chitin derivatives and/or chitosan derivatives, kneading the mixture well and applying an ionizing radiation to the kneaded mixture. The chitin derivatives having a crosslinked structure are hydrogels and the chitosan derivatives having a crosslinked structure are hydrogels having antimicrobial activity.
Hydrogels can hold a large amount of water within the three-dimensional network structure generated by crosslinking with radiation. The retained water will not seep out under slight pressure. Such hydrogels are already in use as disposable diapers and as humectants in cosmetics.
Carboxymethyl-chitin (CM-chitin) and carboxymethyl-chitosan (CM-chitosan), if they are irradiated in either a solid form or as a dilute (≦5%) aqueous solution, preferentially undergo decomposition and no hydrogels will form. If they are irradiated in a concentrated paste form (that will not flow out if the container is tilted), a crosslinked structure can be introduced and the resulting gel will absorb water upon immersion in it to become a hydrogel. In the case of a CM-chitosan hydrogel, it has been found to have a new feature, antimicrobial activity, in spite of it being a hydrogel.
Hydrogels can be easily obtained by applying an ionizing radiation to aqueous solutions of polyethylene oxides, poly(vinyl alcohol), polyacrylamides, polyvinylpyrrolidone, etc. Being capable of absorbing and holding a large amount of water, hydrogels are used in medical and cosmetics fields as sanitary products (e.g. disposable diapers) and humectants. These hydrogels are primarily made of poly(sodium acrylate) based materials. Used hydrogels are disposed of by incineration, so if they are treated massively, the temperature in the incinerator will drop to cause a potential problem of producing dioxins. Attempts are therefore being made to use hydrogels that decompose in the soil to exert no environmental impact, as exemplified by poly(sodium glutamate) and poly(sodium aspartate) having irradiation-generated crosslinks.
Formaldehyde and glutaraldehyde are conventionally used to create chemical crosslinks in chitin and chitosan. However, since the aldehydes contaminate the working environment or the residual aldehydes may irritate the skin, a safer method of crosslinking is desired.
Hydrogels produced by crosslinking water-soluble polymers absorb a large amount of water, so they are used in sanitary products such as disposable diapers. Typically, zeolite incorporating antimicrobes such as silver are added as an antimicrobial agent. However, accumulation of silver is not preferred from the viewpoint of health. It is therefore required to develop high-molecular weight polymers that themselves have antimicrobial activity.
Chitosan has positive electric charges, so they bind to negatively charged microorganisms and exhibit antimicrobial activity to inhibit microbial growth. Thus, the applicable scope of chitin and chitosan which are currently discarded will expand and their added value will further increase if a safer method of their crosslinking is found and if hydrogels that themselves have antimicrobial activity are developed. However, chitin and chitosan have no compatible solvents and if they are irradiated in a solid state, marked decomposition will occur. Therefore, chitin and chitosan are difficult to process into films or fibers and it is also difficult to crosslink them by radiation.
If the hydrogen in hydroxyl groups in chitin and chitosan is replaced by a hydroxyl or carboxyl group, intermolecular hydrogen bonds sufficiently weaken that chitin and chitosan will come to dissolve in water. As a result of their intensive studies, the present inventors found that carboxymethylated chitin and chitosan (chitin and chitosan derivatives) could be crosslinked when irradiated in a thick paste form. The present invention has been accomplished on the basis of this finding. It was also revealed that CM-chitosan was a hydrogel having a unique feature of presenting antimicrobial activity.